So our Mass goes back, without essential change, to the age when it first developed out of the oldest liturgy of all. It is still redolent of that liturgy, of the days when Cæsar ruled the world and thought he could stamp out the faith of Christ, when our fathers met together before dawn and sang a hymn to Christ as to a God. The final result of our enquiry is that, in spite of unsolved problems, in spite of later changes, there is not in Christendom another rite so venerable as ours. ~Fortescue

Does anybody know if Pius XII has been declared a "Venerable"? In the internet appears as if JPII declared Pius XII a venerable but this is not possible, because his process has not yet reached that the stage of venerable.

He has not been beatified yet. Now, I do not know if the title "Venerable" can be used for those whose process has been opened. In that case, that might be why he is JPII referred to him using that title. Another possibility might be that usually Popes say " ...Our Predecessor, of venerable memory, etc...," which does not imply that Pius XII is already beatified.